1. Field
We describe a frequency divider and, more particularly, a frequency divider with a high speed dual modulus prescaler and an associated method.
2. Related Art
Frequency divider circuits are part of frequency synthesizers. In Radio Frequency (RF) systems, frequency synthesizers generate a local oscillator's frequency to step up or step down a frequency band.
Frequency synthesizers usually include a Phase Lock Loop (PLL), and generate a frequency different from the frequency of an input signal. The PLL is a basic building block of modern electronic systems. As shown in FIG. 1, the PLL circuit includes a phase/frequency detector 100, a charge pump 200, a loop filter 300, a Voltage Controlled Oscillator (VCO) 400, and a frequency divider 500.
The phase/frequency detector 100 generates an up-signal SUP and/or down-signal SDN based on a phase difference between a reference signal SIN and a feedback signal SFEED. The charge pump 200 outputs a signal having a level determined by a state of the up-signal SUP and/or the down-signal SDN. The loop filter 300 removes a high frequency component of the signal provided by the charge pump 200, and provides the input voltage VLF to the VCO 400. The VCO 400 outputs a high frequency signal having a frequency determined by the direct current level of the input voltage VLF. The frequency divider 500 generates the feedback signal SFEED having a low frequency based on the VCO output signal SOUT. The phase/frequency detector receives the feedback signal SFEED from the divider 500.
Downstream circuitry (not shown) use the VCO 400 output signal SOUT for various applications after the PLL circuit is locked. Many embodiments of the frequency divider 500 shown in FIG. 1 currently exist. For example, U.S. Pat. No. 6,696,857 describes the frequency divider shown in FIG. 2. Referring to FIGS. 1 and 2, dual modulus prescaler includes D flip-flops 12 and 14, NMOS transistors MN1 and MN2, a PMOS transistor MP1, and a NAND gate 21.
The dual modulus prescaler of FIG. 2 receives an output signal SOUT from VCO 400 as an input signal to divide the frequency of the output signal SOUT by 4 or 3, and outputs the feedback signal SFEED.
The output signal SOUT clocks the D flip-flops 12 and 14, respectively. The NAND gate 21, the NMOS transistors MN1 and MN2, and the PMOS transistor MP1 control the frequency division ratio of the dual modulus prescaler. When a mode signal MODE has a logic level ‘0’, an output signal of the NAND gate 21 has a logic level ‘1’. As a result, the NMOS transistor MN1 is on and a node B has a logic level ‘0’. At this time, the NMOS transistor MN2 is off, and a node C is not at GND. Accordingly, the D flip-flops 12 and 14 of the dual modulus prescaler divide the frequency of the input signal by 4.
When a mode signal MODE has a logic level ‘1,’ on the other hand, and the output signal SFEED of the D flip-flop 14 has a logic level ‘1’, the output signal of the NAND gate 21 has a logic level ‘0’ and the NMOS transistor MN1 is off. When an inverted output signal of the D flip-flop 14 has a logic level ‘0’, the PMOS transistor MP1 is on, and node B has a logic level ‘1’ that turns on the NMOS transistor MN2. The node C, therefore, is at GND. As a result, the D flip-flops 12 and 14 of the dual modulus prescaler divide the frequency of the input signal by 3. The frequency divider shown in FIG. 2 operates at a speed dependent on the dual modulus prescaler.
The frequency divider's operating speed depends on a delay time associated with the NAND gate 21 and the NMOS transistors MN1 and MN2, since these components together with the PMOS transistor MP1 control the frequency division ratio. The delay time is related to the time until the output signal SFEED of the D flip-flop 14 and the mode signal MODE reach a node C.
The dual modulus prescaler of FIG. 2 is not suitable for a PLL system operating at a high frequency, e.g., in the range of 1 to 10 GHz due to the delay times of the NAND gate 21 and the NMOS transistors MN1 and MN2.
Accordingly, a need remains for a frequency divider having a dual modulus prescaler capable of operating at high frequencies.